Biochemistry

Biochemistry, the chemistry of life, is a delicate balance of anabolism, or synthesis, and catabolism, or degradation. Some familiarity with biochemistry is essential for understanding microbial metabolism and biological processes.

Major classes of biochemicals are carbohydrates, lipids, and proteins. Others are enzymes, nucleic acids, and the like. Some important concepts are:

  1. Reactions are reversible.
  2. Equilibrium can be reached with roughly equal amounts of reactants and products or with proportions ranging from almost all products to almost only reactants.
  3. Reactions can be coupled so that energy from one drives another.
  4. Pathways are sequences of reactions; they may have branches.
Many reactions procede to a lower energy state and are thus favored thermodynamically. Examples are the hydrolysis of starch to glucose, the rearrangement of glucose to form fructose, and the splitting of phosphate groups from other phosphates. However, there must be reactions that are not favored thermodynamically, for example the reverse of these favored reactions. The formation of starch from glucose is extremely important as cells must make this storage compound when times are good to tide them over when nutrients are scarce. Coupling of an unfavored reaction to a favored reaction is nature's way of synthesis. Enzyme cofactors participate in coupled reactions.

Adenosine triphosphate (ATP) is the main energy transfer compound in cells. Its phosphate linkages release considerably more energy during hydrolysis than do simple ester linkages that resemble these phosphate bonds. Some of the key pathways in respiration provide bond energy to adenosine diphosphate (ADP) to convert it to ATP. There are also oxidation-reduction reactions that take the enzyme cofactors nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) to their reduced forms. A series of reactions involving such other cofactors as flavins reoxidize NADH and NADPH to NAD and NADP while forming ATP. However, these reactions require oxygen as the ultimate electron acceptor and do not procede under anaerobic conditions. As a general rule, anaerobic processes give little energy to the cells and organic compounds tend to accumulate. Aerobic processes give more complete oxidation and roughly fifteen times as much energy per glucose molecule as do anaerobic processes.

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